Relativity holds that time and space are related, they combine to form four-dimensional space-time coordinates and participate in physical processes, so time and space are related, and space is no longer the whole space.
A typical example is that the time around a massive object will slow down. Last year, there was a very intuitive demonstration in Interstellar. There is also a saying that high speed will slow down time, and there is also a saying that "the moving clock will slow down".
The special relativity principle of extended data (special covariance principle): All inertial reference frames have the same weight, that is, the form of physical laws is the same in any inertial reference frame. This means that the laws of physics are the same for an observer who is still in the laboratory and an electron who is moving at a high speed and at a constant speed relative to the laboratory.
Principle of constant light speed: the light speed in vacuum is constant in any reference frame, which can be expressed by the geometric language that the world line of photons in time and space is always light-like. It is precisely because of the experimental nature of photons that the length unit "meter" is defined by "the distance traveled by light in vacuum 1/299792458 seconds" in the international system of units.
Relativity is mainly useful in two aspects: one is high-speed motion (high speed equivalent to the speed of light), and the other is strong gravitational field.
References:
Baidu encyclopedia-theory of relativity